Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder air intake port (port fuel injection). Multi-fuel engine systems can use both port and direct injection with different fuel types provided to the different injectors. For example, direct injection of ethanol fuel may be used with port injection of gasoline fuel. Therein, the direct injection of the alcohol fuel may take advantage of the increased charge cooling effects of the alcohol fuel's higher heat of vaporization and increased octane. This helps to address knock limitations, especially under boosted conditions. Direct injection of fuel may also be used during an engine cold start to operate overall leaner than stoichiometry but richer near the spark plug for robust cylinder combustion. Further, the port injection of the gasoline fuel may take advantage of the higher energy density of the gasoline fuel and improved fuel vaporization at lower engine temperatures as compared to that of alcohol fuel.
However the inventors herein have recognized that different ignition system energy outputs and spark duration may be required when injecting different fuels. Further, different ignition system energy outputs and spark duration may be required based on the injection mode (direct or port). Managing the ignition energy output and spark duration may become even more complicated as transitions are made between fuels and injection modes. As such, if the ignition energy output and spark duration are not adjusted to the appropriate level, various issues may occur such as incomplete combustion, over-usage of spark energy, component durability issues, fuel economy and emissions degradation.
In one example, some of the above issues may be at least partly addressed by a method for an engine comprising: adjusting an ignition coil dwell time for a cylinder spark event based on a ratio of fuel received in the cylinder via port injection relative to direct injection. The adjusting may be further based on the type of fuels received via port injection relative to direct injection. Further still, the adjusting may be based on the split of the direct injection including the ratio of fuel direct injected in an intake stroke relative to a compression stroke. In this way, ignition energy and spark duration may be adjusted for different fuel types and different types of injection.
As an example, for a spark event of a cylinder combustion event, ignition coil parameters may be adjusted based on the types of fuel received in the cylinder during the cylinder combustion event, as well as the relative proportion of total cylinder fueling received via a port injector and a direct injector. For example, the cylinder may receive a first amount of a first fuel via the port injector while receiving a second amount of a second, different fuel via the direct injector for the given combustion event. The engine system may be configured with a single ignition coil wherein a charging current is applied for a dwell time on the ignition coil, after which the coil is discharged to the spark plug to initiate the spark event. Herein, the dwell time for which the charging current is applied may be adjusted based on the first and second fuels and based on the ratio of fuel delivery via port and direct injectors. As an example, as the amount of fuel delivered via the direct injector increases, the dwell time may be increased due to charge cooling from the direct injection making in-cylinder compression pressures slightly higher. As such, as the dwell time is increased, the peak charging current applied to the coil increases, increasing the ignition energy output of the culminating spark event.
In an alternate example, the engine system may be configured with a dual ignition coil wherein a first charging current is applied for a first dwell time on a first ignition coil, and a second charging current is applied for a second dwell time on a second ignition coil. After the first dwell time has elapsed, the first coil is discharged to the spark plug to initiate the spark event. While the first coil is discharging, and after a delay since the discharging of the first coil, the second coil is discharged to the spark plug. Herein, the dwell time for each of the first and second ignition coils may be adjusted based on the first and second fuels and based on the ratio of fuel delivery via port and direct injectors. As an example, as the amount of fuel delivered via the direct injector increases, the dwell time of the first and second ignition coils may be increased. In addition, a delay between discharging of the two coils may be decreased to. As elaborated herein, the ignition energy requirement may depend on various factors of the combustion chamber and may vary engine to engine. In general, ignition energy may be increased when the direct injection fuel fraction is higher due to increased charge and compression pressure during discharge. If the direct injected fuel fraction is split into multiple injections, e.g., an intake stroke and a compression stroke direct injection, and if the compression stroke injection is used in a stratified manner, a favorable air-fuel ratio near the spark plug may translate into a lower ignition energy requirement, as well as a lower need for a second discharging.
The adjusting of the dwell time and the delay may also be based on the fuel type delivered via the port injector and the direct injector. For example, the adjusting may be based on a difference in alcohol content of the fuels, or an octane rating of the fuels. Thus, the dwell time for each ignition coil may be lower when the port injected fuel includes gasoline and the direct injected fuel includes E85 as compared with when the port injected fuel includes gasoline and the direct injected fuel includes E10. Furthermore, the dwell time may be adjusted as a function of fuel reactivity (or ignitability). As such, higher reactivity fuels may have a lower ignition energy demand.
In this way, spark energy for a combustion event may be varied by adjusting the dwell command of an ignition coil based on cylinder fueling, the ignition energy output during a spark event in the cylinder may be better managed. In particular, the ignition energy output may better match the ignition output required for combustion of the given combination of fuels and injection types. By increasing the ignition output as fuel delivery via direct injection increases, and fuel delivery via port injection decreases, power consumption of the ignition event is improved. In addition, component durability is increased.
As such, this enables robust combustion with minimal energy usage. In addition, durability of ignition components such as ignition coils and spark plugs is not compromised.
Overall, cylinder combustion is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.